1. Field of the Invention
This invention relates to methods of treating waste gases to remove sulfur oxides and more particularly to removing sulfur oxides from waste gases by scrubbing using a reactive absorbent. The invention also relates to improved methods of removing sulfur oxides from waste gases generated in the manufacture of portland cement.
2. Brief Description of the Prior Art
Sulfur oxides are ubiquitous contaminants in waste gases from industrial processes. They are produced in the combustion of fossil fuels, especially coal and oil, which is the source of process heat for most industrial activity. Because these gaseous oxides are notorious constituents of air pollution and contribute substantially to the formation of acid rain, the emission of sulfur oxides, i.e., sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3), is strictly regulated, and the restrictions on such emissions are expected to become more stringent in the future.
Because of the importance of removing sulfur oxides from industrial waste gases, a wide variety of techniques has been developed to treat such waste gases to reduce or eliminate the emission of sulfur oxides. Among such techniques are those employing adsorption, chemical reaction, condensation, and absorption.
Absorption techniques typically involve contacting the waste gas stream with a liquid in which the gas to be removed is soluble or with which it reacts to form a dissolved solute or a precipitate. The continuous phase in such a contact arrangement may be the gas or the liquid. In absorption of acidic gases such as sulfur oxides it is conventional to use an alkaline liquid as the absorbent. Among the alkaline agents used in absorption liquids for sulfur oxides in waste gas streams are soluble alkalis such as sodium hydroxide and calcium hydroxide. Often the calcium hydroxide is provided by slurrying lime (calcium oxide) with water.
In most conventional types of scrubbing apparatus a stream of waste gas containing sulfur oxides is contacted with an alkaline absorbing liquid in co-current or countercurrent fashion. In order to enhance the surface area of contact between the liquid and the gas, the liquid may be sprayed into the gas stream or trickled over a bed of packing. Both cocurrent and countercurrent contacting arrangements are used.
One well-developed gas scrubbing apparatus for providing intimate contact between a waste gas and a scrubbing liquid is the reverse jet scrubber disclosed in U.S. Pat. No. 3,803,805, to Low. In such apparatus, waste gas is passed through a conduit at a relatively high velocity while a jet of scrubbing liquid is directed countercurrently and generally coaxially to the flow of gas. The gas velocity is sufficiently high so that the jet is turned back and carried along with the gas stream to subsequent processing operations. The intimate mixture of forward flowing waste gas and reverse jet liquid stream produces a froth zone in the conduit which is conducive to intimate contact between the waste gas and the scrubbing liquid. This intimate mixing assures a thorough wetting of particles in the waste gas stream so that they may be easily collected downstream and is also conducive to absorption of gases in the waste stream into the scrubbing liquid. In particular, acidic gases such as sulfur oxides can be absorbed in a reverse jet spray, especially if the liquid contains an alkaline agent to react with the acidic gas. However, the only alkaline agent exemplified in the Low patent for removal of sulfur dioxide is sodium hydroxide in dilute aqueous solution. U.S. Pat. No. 3,803,805 does not disclose the use of limestone in a reverse jet absorption system.
U.S. Pat. No. 4,102,982, to Weir, discloses a process for removing pollutants such as sulfur dioxide from stack gases by passing the waste gases through a substantially horizontal conduit while spraying a scrubbing liquid transverse to the flow of gas. For removing acidic pollutants such as sulfur dioxide a scrubbing solution containing lime may be used. Weir discloses that the use of limestone in conventional wet scrubbers for removing sulfur dioxide from waste gases has resulted in a lower efficiency of removal.
U.S. Pat. No. 4,374,813, to Chen, discloses an improved reverse jet scrubber for removing acidic pollutants form waste gases. However, Chen discloses only the use of a caustic solution as the scrubbing liquid.
U.S. Pat. No. 4,576,803, to Hegemann et al., discloses scrubbing flue gases with scrubbing solutions containing lime which may include calcium carbonate as well. However, Hegemann does not disclose maintaining a high efficiency in scrubbing flue gases with scrubbing solutions containing limestone.
Hamm, H., et al., "The Two-stage Knauf-Research-Cottrell Process for Flue Gas Desulphurization as Exemplified by the Franken Power Station", Zem.-Kalk-Gips (ZKG), Ed. B (1982), Vol. 35(6), pp. 313-317, discloses scrubbing flue gas from a coal-fired power station with a slurry of finely divided limestone (particle size less than 90 micrometers) in a relatively complex two-stage countercurrent spray tower. The process disclosed in this reference achieves removal of about 90% of the sulfur dioxide in the flue gas.
Thus, the prior art has not considered limestone to be an efficient material for absorbing sulfur dioxide from waste gases, although it has been used in some cases for economic reasons in spite of its low efficiency.
Accordingly a need has continued to exist for a simple method of removing sulfur oxides from a waste gas stream that exhibits high efficiency with the use of economical absorbents.